Composition and process for prepainting treatment of plastics

ABSTRACT

This invention relates to particular aqueous coating compositions comprising particular organic acids and processes of using these compositions to treat plastic moldings.

TECHNICAL FIELD

The invention relates to a water-based liquid composition for treatingplastics, particularly plastic moldings, more particularly those made ofpolypropylene, polyurethane, and the like, that are used, e.g., as bodypanels, bumpers, or the like in automobiles. The invention is especiallysuited to prepainting treatment of such plastics. The invention isaccordingly described below primarily with reference to prepaintingtreatment for plastic moldings, but more generalized application toplastic surfaces is within the scope of the invention.

BACKGROUND ART

The plastic moldings used for automotive bumpers, etc., usually arecurrently manufactured by injection molding. In the case of resins thatare prone to stick to the mold, such as polyurethane, the mold surfaceis usually coated prior to actual molding with a wax-based externalrelease agent, and a metal soap (for example, zinc stearate), whichfunctions as an internal release agent, is also frequently added to suchresins in advance of molding. Various additives other than internalrelease agent often are also blended into resins for the purpose ofimproving the strength, improving paint adherence, improving theresistance to aging, and the like. As a result, during the moldingprocess the strongly hydrophobic internal additives and of course anyinternal release agent bleed onto the surface to some degree. Thepresence on the surface of substances such as external release agent andbled-out internal additives impairs the post-painting adherence by thepaint film and also causes paint crawling and thereby compromises theappearance of the paint film. The preparatory treatment that precedespainting must therefore not only remove contaminants adhering aftermolding, such as dust and oil, but must also remove substances such asexternal release agent and internal additives that have bled onto thesurface.

The treatment of plastic moldings prior to painting has heretoforeconsisted mainly of the use of solvent cleaners such as1,1,1-trichloroethane, freons, and the like. This type of treatment candissolve and remove release agents and also has an excellent operatingefficiency due to its ease of drying. However, as is well known,environmental considerations have made the development of substitutepretreatment methods a matter of urgency.

Cleaning with acidic and alkaline water-based cleaners and cleaning withemulsified solvent cleaners have been investigated as substitutepretreatment methods. These methods differ from solvent cleaning in thatthey require a rinse step and therefore require a drying step (seeJapanese Laid Open Patent Application Numbers Hei 4-27472 27,472/1992!and Hei 5-39499 39,499/1993!).

However, even after release agent on the surface has been removed bycleaning, drying must be conducted at low temperatures since internalrelease agent and strongly hydrophobic internal additives bleed onto thesurface during the drying step. This lengthens the drying time and thusnecessitates the use of large-scale equipment. In addition, sinceinternal release agents and strongly hydrophobic internal additivesgradually bleed onto the surface even at room temperature, thepaintability also becomes impaired when a lengthy period of time isallowed to elapse between cleaning and painting. This requires that thecleaned molding be promptly painted.

As described above, research has heretofore focused on the removal ofinternal additives once they have bled onto the surface of the plasticmolding from within the molding; however, up to now in the knowledge ofthe applicants there have been no investigations into the inhibition ofbleed-out by internal additives onto the surface.

DISCLOSURE OF THE INVENTION Problems to Be Solved by the Invention

The present invention is directed to an inhibition of the surface bleedby internal plastic additives that occurs with elapsed time and duringthe drying step in the interval after the water-based treatment ofplastic moldings preparatory to their painting. More specifically, thepresent invention takes as its objects an increase in the dryingtemperature in order thereby to support equipment downsizing, animprovement in the paintability (e.g., paint crawling, paint adherence),and/or in particular a stabilization of the paintability through aninhibition of timewise changes in the paint-receiving surface.

SUMMARY OF THE INVENTION

The inventors discovered that treating the surface of a plastic moldingwith an aqueous solution containing specific organic acids results in aninhibition of the surface bleed by internal plastic additives and thusin an inhibition of timewise changes in the paint-receiving surface anda stabilization of the paintability. In specific terms, one embodimentof the present invention is a water-based liquid composition fortreating plastic moldings prior to the painting thereof, wherein saidcomposition is an aqueous solution that, in addition to water,comprises, preferably consists essentially of, or still more preferablyconsists of at least 0.05 weight % of at least one selection fromcarboxylic acids that do not contain any terminal alkyl group in theirmolecular structure and hydroxycarboxylic acids that do not contain anyterminal alkyl group in their molecular structure; optionally, thecompositions according to the invention may also contain surfactants,other builders, and or other acids. A process according to the inventionincludes at a minimum a step of contacting a plastic surface with acomposition according to the invention as described herein.

DESCRIPTION OF PREFERRED EMBODIMENTS

The most important characteristic of the water-based composition oragent in accordance with the present invention for treating plasticmoldings prior to their painting (hereinafter abbreviated as theprepaint treatment agent) is that said agent is an aqueous solution thatcontains as its essential component at least one selection fromcarboxylic acids that do not contain any terminal alkyl group in theirmolecular structure and hydroxycarboxylic acids that do not contain anyterminal alkyl group in their molecular structure.

For the purposes of the present invention, the absence of any terminalalkyl group from the molecular structure means that no group having theformula --C_(n) H_(2n+1) is present in the molecule.

Terminal alkyl-free carboxylic acids are specifically exemplified byoxalic acid, malonic acid, succinic acid, glutaric acid, and adipicacid.

Terminal alkyl-free hydroxycarboxylic acids are specifically exemplifiedby glycolic acid, malic acid, tartaric acid, and citric acid.

The aqueous solution preferably contains at least 0.05 weight % and morepreferably at least 0.1 weight % of the one or more selections fromterminal alkyl-free carboxylic acids and terminal alkyl-freehydroxycarboxylic acids. The surface bleed of internal additives fromwithin the plastic molding usually is not adequately inhibited at below0.05 weight %. Independently, for reasons of economy, the aqueoussolution according to the invention preferably contains not more than 5,or more preferably not more than 2.2 weight % of these acids.

The aqueous solution in accordance with the present invention asdescribed above (=aqueous solution that contains at least one selectionfrom terminal alkyl-free carboxylic acids and terminal alkyl-freehydroxycarboxylic acids) functions to inhibit bleed-out by internaladditives present in the plastic molding.

However, a surfactant normally must also be used to remove othercontaminants, for example dust and oil, that adhere after molding. Saidsurfactant may be used in the prepaint treatment agent of the inventionin combination with organic acid as specified above for the invention.Alternatively, contaminants such as dust and oil may first be removedusing a solution that contains surfactant and treatment may then becarried out using a surfactant-free prepaint treatment agent inaccordance with the invention.

No particular restrictions apply to the surfactant, and its type andquantity of use should be selected as a function of the particularconditions, such as the nature of the resin, the status of adhesion ofthe dust and oil, etc.

In addition to surfactant, the prepaint treatment agent of the inventionmay contain other builders.

Plastic moldings can be treated with the prepaint treatment agent of theinvention by spraying or immersion. The bath temperature is not criticalto the effect of the invention, i.e., the inhibition of bleed-out byinternal additives. However, when surfactant is present in the prepainttreatment agent of the invention and this agent is relied on for generalcleaning as well as for the inhibition of bleed-out, the temperatureshould be selected as a function of such considerations, generally knownin the art, as the adhesion status of any oil, dust, or the like inorder to accomplish their removal by the surfactant.

Treatment with the water-based pretreatment agent of the invention isbelieved to increase the surface free energy of plastic surfaces. Thiseffect is not destroyed even by water washing after treatment and is notdegraded by drying or by the passage of time after drying. It is thoughtthat the inhibition of surface bleed by internal additives is caused bythis increase in the surface free energy of the plastic surface. Theorganic acids specified for the present invention do not contain anyterminal alkyl group in the molecule. In fact, however, terminalalkyl-containing organic acids do appear to produce an increase in thesurface free energy immediately after treatment, but with thesecompounds the surface free energy is reduced by drying and by the elapseof time. It is for this reason that the use of alkyl-containing organicacids does not yield an adequate improvement in the paintability.

Moreover, the modification of surface properties as a result ofexecution of the prepaint treatment of the invention results in animproved wettability by aqueous solutions and thus in improved dryingcharacteristics.

The invention and its benefits may be further appreciated byconsideration of the following Examples (according to the invention) andComparative Examples (not according to the invention).

Examples and Comparative Examples

1. Test materials

amine-cured polyurethane (bumper material) external release agent:oxidized wax type internal release agent: zinc stearate

hard polypropylene (bumper material)

2. Composition of the pretreatment agents

Pretreatment agent 1

(corresponds to a prepaint treatment agent of the invention)

    ______________________________________                                        sulfuric acid:          3 wt %                                                polyoxyethylene nonylphenyl ether                                                                     0.5 wt %                                              organic acid            see Table 1                                           water                   remainder                                             ______________________________________                                    

Pretreatment agent 2

(not a prepaint treatment agent of the invention)

    ______________________________________                                        sulfuric acid           3 wt %                                                polyoxyethylene nonylphenyl ether                                                                     0.5 wt %                                              (HLB: 12)                                                                     water                   remainder                                             ______________________________________                                    

Pretreatment agent 3

(corresponds to a prepaint treatment agent of the invention)

    ______________________________________                                        organic acid         see Table 1                                              water                remainder                                                ______________________________________                                    

Pretreatment agent 4

(corresponds to a prepaint treatment agent of the invention)

    ______________________________________                                        polyoxyethylene nonylphenyl ether                                                                     0.5 wt %                                              (HLB: 12)                                                                     organic acid            see Table 1                                           water                   remainder                                             ______________________________________                                    

3. Treatment methods

Method A:

    ______________________________________                                        treatment with pretreatment agent 1                                                               (180 sec, 80° C.)                                     ↓                                                                   water wash          (30 sec, ambient temperature)                                ↓                                                                   wash with pure water                                                                              (30 sec, ambient temperature)                                ↓                                                                   drain and dry       (10 min, 80° C. or 120° C.)                 ______________________________________                                    

Method B:

    ______________________________________                                        treatment with pretreatment agent 2                                                               (180 sec, 80° C.)                                     ↓                                                                   water wash          (30 sec, ambient temperature)                                ↓                                                                   treatment with pretreatment agent 3                                                               (10 sec, ambient temperature)                                ↓                                                                   wash with pure water                                                                              (30 sec, ambient temperature)                                ↓                                                                   drain and dry       (10 min, 80° C. or 120° C.)                 ______________________________________                                    

Method C:

    ______________________________________                                        treatment with pretreatment agent 4                                                               (180 sec, 80° C.)                                     ↓                                                                   drain and dry       (10 min, 80° C. or 120° C.)                 ______________________________________                                    

(Spraying was used for treatment with the pretreatment agent and for thewater washes.)

4. Painting

After pretreatment, the sample was held as specified below and thenpainted.

Holding conditions 1: painted within 12 hours after pretreatment.

Holding conditions 2: painted on the fifth day after pretreatment.

Primers:

In the case of the amine-cured polyurethane: Soflex® 1000 from KansaiPaint Kabushiki Kaisha, baking for 15 minutes at 110° C., filmthickness=15 to 20 micrometers;

In the case of the hard polypropylene: Soflex® 2500 from Kansai PaintKabushiki Kaisha, baking for 10 minutes at 100° C., film thickness=15 to20 micrometers

Top coat:

The following was used as the top coat for both sample materials:Soflex® 1200 from Kansai Paint Kabushiki Kaisha, baking for 20 minutesat 120° C., film thickness=35 to 40 micrometers.

5. Test methods used for evaluation

(Contact angle)

After the execution of the pretreatment and drain and dry, or for othersamples after completion of the holding period in addition, a drop ofdistilled water was placed on the surface of the sample and the contactangle was measured with a goniometer-type contact angle instrument. (Thevalue measured immediately after drain and dry only is designated"initial" below.) Lower contact angles are indicative of lowerinterfacial energies between water and the treated solid surface andhence of better paintabilities.

(Adhesion after Water Soak)

After a holding period of 24 hours after painting, a checkerboardpattern of 100 squares, each 2 mm on a side, was cut in the paintsurface using a sharp cutter. After an ensuing immersion for 240 hoursin de-ionized water at 40° C., cellophane tape was applied to thecheckerboard and then sharply peeled off and the number of residualpaint film squares was counted. A larger number of residual squares isindicative of a better adhesion.

(Paint crawling test)

The appearance of the primer was visually evaluated after itsapplication and was scored on the following three-level scale.

+ +: no primer crawling was observed

+: orange-peel crawling was observed

x: crawling is observed to such a degree that the substrate has becomevisible

The results of the performance evaluations are reported in Table 1, andthese results support the following conclusions:

1. The contact angle, whether measured initially or after the holdingperiod, was smaller and its variation was less in Examples 1 to 6 of theinvention than in Comparative Examples 1 to 7. This was true even whenhigh temperature paint-baking conditions were employed. The adhesionafter water soak was excellent in Examples 1 to 6, and no paint crawlingwas observed in Examples 1 to 6. These excellent property valuesconfirmed the inhibition of internal additive bleed from the plasticmoldings.

2. In Examples 1 to 6 of the invention, the differences among treatmentmethods A, B, and C did not cause differences among the initial contactangle values, the timewise variation in the contact angle values, theadhesion after water soak, or paint crawling. This indicates that thereis no difference between use of the surfactant in the prepaint treatmentagent of the invention and preliminary treatment with a solution thatcontains only surfactant followed by treatment with surfactant-freeprepaint treatment agent in accordance with the present invention.

                  TABLE 1                                                         ______________________________________                                        TEST VARIABLES AND RESULTS                                                    OF THE PERFORMANCE EVALUATIONS                                                                     Initial                                                  Pre-                 Contact Angle                                                                            Drain/                                        treat-   Organic Acid                                                                              in Degrees Dry   Holding                                 ment              %      On:      Temp.,                                                                              Condition                                   Code   Type     Conc.                                                                              PU   PP    °C.                                                                          Code                              ______________________________________                                        Ex. 1 A      Succinic 0.5  66   97    120   1                                 Ex. 2 B      Malic    0.1  67   99     80   2                                 Ex. 3 C      Tartaric +                                                                             0.3  64   83    120   1                                              Citric                                                           Ex. 4 C      Malonic  1.0  65   85     80   2                                 Ex. 5 A      Oxalic    0.05                                                                              68   100   120   1                                 Ex. 6 B      Citric   2.0  65   99    120   1                                 C.Ex. 1                                                                             A      none     --   82   98    120   1                                 C.Ex. 2                                                                             A      none     --   83   101    80   2                                 C.Ex. 3                                                                             B      Malic     0.03                                                                              80   99    120   1                                 C.Ex. 4                                                                             B      Tartaric  0.01                                                                              79   98     80   2                                 C.Ex. 5                                                                             A      Methyl   1.0  67   100   120   1                                              Succinic                                                         C.Ex. 6                                                                             B      Lactic   2.0  68   98    120   1                                 C.Ex. 7                                                                             C      Acetic   0.5  65   102    80   2                                 ______________________________________                                        For Polyurethane:       For Polypropylene                                                   Adhesion              Adhesion                                                After    Paint        After  Paint                                    Contact Water    Crawl- Contact                                                                             Water  Crawl-                                   Angle, °                                                                       Soak     ing    Angle, °                                                                     Soak   ing                                ______________________________________                                        Ex. 1 66      100      ++      98   100    ++                                 Ex. 2 68      100      ++     100   100    ++                                 Ex. 3 64      100      ++      82   100    ++                                 Ex. 4 64      100      ++      86   100    ++                                 Ex. 5 69      100      ++     101   100    ++                                 Ex. 6 65      100      ++      96   100    ++                                 C.Ex. 1                                                                             106     29       X      112   94     +                                  C.Ex. 2                                                                             92      82       +      108   96     +                                  C.Ex. 3                                                                             98      67       X      110   95     +                                  C.Ex. 4                                                                             86      94       +      110   97     +                                  C.Ex. 5                                                                             102     32       X      109   94     +                                  C.Ex. 6                                                                             98      56       X      110   93     +                                  C.Ex. 7                                                                             85      96       +      109   97     +                                  ______________________________________                                         Notes for Table 1                                                             "PU" = Polyurethane;                                                          "PP" = Polypropylene;                                                         "Ex." =  Working! Example;                                                    "C.Ex." = Comparative Example                                                 Values for contact angle in columns in which the words "Contact Angle" in     the column heading are not immediately preceded by the word "Initial" wer     measured on the treated surfaces after pretreatment, followed by drain an     dry and then followed by the holding period, but before painting the          surface.                                                                 

3. The values for the property set (initial contact angle value,timewise variation in the contact angle value, adhesion after watersoak, and paint crawling) in Comparative Examples 1 and 2 (with noaddition of organic acid) were inferior to those of Examples 1 to 6.

4. Comparative Examples 3 and 4 did employ terminal alkyl-free organicacids, but at concentrations of 0.01 to 0.03 weight %, or lower than the0.05 to 2.0 weight % used in Examples 1 to 6. The values for theproperty set (initial contact angle value, timewise variation in thecontact angle value, adhesion after water soak, and paint crawling) wereagain inferior. This indicates that 0.05 weight % is a critical organicacid concentration value with regard to the development of the effectsof the invention.

5. Comparative Examples 5 to 7, which used terminal alkyl-containingorganic acids, gave approximately the same initial contact angle valuesas Examples 1 to 6, but had inferior values for the timewise variationin the contact angle, adhesion after water soak, and paint crawling.This was true despite use of the terminal alkyl-containing organic acidsat concentrations of 0.5 to 2.0 weight %.

Benefits of the Invention

The surface bleed of internal additives present in plastic moldings isinhibited by treatment with the water-based prepaint treatment agent inaccordance with the present invention. This results in the followinghighly desirable effects: high-temperature drying becomes possible,which supports a shortening of the drying time and equipment downsizing;and changes in the surface with elapsed time are inhibited; thisstabilizes the paintability and improves the workability.

The invention claimed is:
 1. A process for treating a surface of aplastic material by contacting said surface, for a sufficient time at asufficient temperature to decrease the spreading angle of water on saidsurface from the value of the spreading angle of water on said surfacebefore such treating, with an aqueous solution consisting essentially ofwater and(A) a total of at least 0.05% by weight of one or more acidsselected from the group consisting of carboxylic acids that do notcontain any terminal alkyl group in their molecular structures, andhydroxycarboxylic acids that do not contain any terminal alkyl group intheir molecular structures; and, optionally, one or more of (B) acomponent selected from acids that are not part of component (A) and (C)a component of surfactants not part of components (A) or (B);saidaqueous solution being free of an amount, sufficient to cause an initialincrease in surface energy of the surface of plastic materials treatedto be reduced by drying and elapse of time, of acid molecules thatcontain a terminal alkyl group in their molecular structures.
 2. Aprocess according to claim 1, wherein the plastic surface is that of anarticle produced by injection molding.
 3. A process according to claim2, wherein the amount of component (A) in the aqueous solution used fortreatment is at least 0.1 weight %.
 4. A process according to claim 1,wherein the amount of component (A) in the aqueous solution used fortreatment is at least 0.1 weight %.
 5. A process according to claim 4,wherein component (A) is selected from the group consisting of oxalic,malonic, succinic, glutaric, adipic, glyclolic, malic, tartaric, andcitric acids.
 6. A process according to claim 3, wherein component (A)is selected from the group consisting of oxalic, malonic, succinic,glutaric, adipic, glyclolic, malic, tartaric, and citric acids.
 7. Aprocess according to claim 2, wherein component (A) is selected from thegroup consisting of oxalic, malonic, succinic, glutaric, adipic,glyclolic, malic, tartaric, and citric acids.
 8. A process according toclaim 1, wherein component (A) is selected from the group consisting ofoxalic, malonic, succinic, glutaric, adipic, glyclolic, malic, tartaric,and citric acids.
 9. An aqueous solution, suitable for use as aprepainting treatment for a molded plastic surface, that consistsessentially of water and:(A) a total of at least 0.05% by weight of oneor more acids selected from the group consisting of carboxylic acidsthat do not contain any terminal alkyl group in their molecularstructures and hydroxycarboxylic acids that do not contain any terminalalkyl group in their molecular structures, and (B) a surfactantcomponent not a part of component (A) and, optionally, (C) a componentselected from acids that are not part of component A or component B,saidaqueous solution being free of an amount, sufficient to cause an initialincrease in surface energy of the surface of plastic materials treatedto be reduced by drying and elapse of time, of acid molecules thatcontain a terminal alkyl group in their molecular structures.
 10. Anaqueous solution according to claim 9, wherein the amount of component(A) in the aqueous solution is at least 0.1 weight %.
 11. An aqueoussolution according to claim 10, wherein component (A) is selected fromthe group consisting of oxalic, malonic, succinic, glutaric, adipic,glyclolic, malic, tartaric, and citric acids.
 12. An aqueous solutionaccording to claim 9, wherein component (A) is selected from the groupconsisting of oxalic, malonic, succinic, glutaric, adipic, glyclolic,malic, tartaric, and citric acids.